1. Field of the Invention
The present invention relates to an image forming apparatus applicable to copying machines, facsimile apparatuses, printers and the like.
2. Description of the Related Art
In the conventional electrophotographic image formation process, pixel position shift occurs due to some factors. In a microscopic view, the pixel position shift is positional shift of one toner particle, i.e., scatter of toner, movement faithfully corresponding to electric field of a latent image, or transfer failure; in a macroscopic view, the pixel position shift is a systematic failure as color shift in a copying machine and a printing apparatus.
First, a description will be made on a problem in transfer of toner onto a printing medium by an electric charge q held by the toner and an electrostatic force F=qxc2x7E applied among a photoreceptor body, developing electrodes, a sheet, and an intermediate transfer body.
FIG. 34A shows a spatial electric field caused between a well-regulated rectangular latent image on the photoreceptor body and developing electrodes (The 104th issue of the society of Electrophotography of Japan, Vol. 32 No. 3., 1993). An equipotential line is not completely parallel to the surfaces of the photoreceptor, the developing electrodes and the like, since the photoreceptor body and the developing electrodes are not complete parallel electrodes and an electric force line deflects due to edge effect of edge portions of the rectangular latent image. FIG. 34 shows a profile of contrast potential in case of shift from the center of the latent image in a process direction (photoreceptor-body rotation direction) X and in an object electrode direction (gap) Y. Note that latent images having a uniform charge density of 100 xcexcm in length in the process direction are provided at 100 xcexcm intervals on the photoreceptor body.
FIG. 35 shows the distribution of contrast potential upon scanning in the direction Y. In FIG. 35, a solid line a denotes a position at the center of the central latent image; a dotted line b, an intermediate position between the central latent image and its adjacent latent image; and an alternate long and short dashed line c, the result of scanning at a point 70 xcexcm away from the outer end of the outermost latent image. These calculation results show that the maximum point of the contrast potential lowers as it becomes away from the center of the latent image in the process direction, and the point is becoming shifted between the photoreceptor body and the object electrode. In an actual developing process, development is not started at the latent image center (X=0), but at a point where the potential reaches an effective electric-field threshold value (an effective electric-field threshold value determined by the electric field of the photoreceptor body, the electric charge of toner, and the bias of developing electrode) within a developing nip. Accordingly, the result of development is similar to development shifted in the direction X as shown in FIG. 35.
The toner receives an electrostatic force for transfer and movement, however, as the contrast potential is low and the development is made at the shifted position, the toner cannot be brought to the original pixel position. This is an inherent problem of electrophotographic technique based on the electrostatic dynamics.
On the other hand, in the macroscopic view, the above technique has problems of color shift and color registration. In a low frequency band, color shift occurs, while in a high frequency band, regular stripes (hereinafter referred to as xe2x80x9cbandingxe2x80x9d) occurs in a process direction, greatly depending on the rotation or moving precision (hereinafter referred to as xe2x80x9cmotion qualityxe2x80x9d) of a photoreceptor body, a transfer drum and a sheet conveyance belt generally used in copying machines and printers. The color shift and banding due to bad motion quality appear in a highly precise halftone representation or color characteristic representation as a defect of the image, which greatly degrades the image quality. For this reason, in the conventional copying machines and printers, feedback or feedforward control is performed, or the mechanical precision of constituent parts and assembling is improved so to attain high motion quality in the photoreceptor body and the conveyance belt. Further, various problems of time variation, environmental change, especially change in light quantity of recording-information writing unit due to temperature change, and shift of synchronism among the respective mechanical structures (e.g., in a tandem type copying machine or printer, the ratio between the diameter of a roller for conveying a conveyance belt and the distance between the roller and a photoreceptor body is set to be an integer, however, it changes due to the apparatus frame and temperature expansion of the roller), must be solved. The above problems cause enlargement in apparatus size, complication and a high cost, and poses a limitation on the conventional electrophotographic technique as a printing technique so as not to lower reliability.
However, in the field of printing technique, by solving the problem of pixel position shift, high precision is attained at a cost of high apparatus price. Basically, as a plate is used, pixel position shift in an image writing process as in the electrophotographic printing does not occur. Further, color printing of yellow, magenta, cyan and black is performed in a stable manner from a plate, then a blanket, to a print sheet, by using a physical phenomenon xe2x80x9cthixotropyxe2x80x9d characteristic of printing ink. Further, the plate, the blanket, a sheet support and conveyance member, having high reliability and durability, are driven by high-precision helical gears and the like. Further, the pixel position shift in high quality printing is 15 to 30 xcexcm on an average by performing pixel-position shift regulation by an expert operator. However, as the enlargement of apparatus size and high price cannot be avoided, this printing technique is not sufficiently applicable to on-demand printing as a printing technique for business use or office use.
A printing technique Oce3125C, reported in CeBIT ""96, is known. This technique visualizes toner by bias development without an electrostatic latent image on a photoreceptor body. FIG. 36 shows a cross section of an imaging drum. Ring electrodes 70 are provided in 400 dpi in an insulating layer 71 in its axial direction. The surface of the imaging drum is covered with a dielectric layer 69.
FIG. 37 shows the construction of the entire apparatus. Alphabets xe2x80x9cKxe2x80x9d, xe2x80x9cBxe2x80x9d, xe2x80x9cRxe2x80x9d, xe2x80x9cGxe2x80x9d, xe2x80x9cMxe2x80x9d, xe2x80x9cCxe2x80x9d and xe2x80x9cYxe2x80x9d added to respective reference numerals denote respective colors. When a part is representatively used in the description, such alphabet will be omitted. Each imaging drum 72 has a matrix-arranged fixed electrodes for transmitting printing information from a slip ring (not shown). Toner moves from a developing roller 74, to which an electric field is applied in accordance with the printing information, to the imaging drum 72 and forms an image. The imaging drums 72, i.e., black imaging drum 72B to yellow imaging drum 72Y, are tandem-arranged around a transfer drum 73, and developing rollers 74, i.e., black developing roller 74B to yellow developing roller 74Y, are arranged at the respective imaging drums. A printing process starts with development. The respective seven color toner images are sequentially formed, each in single layer, on the imaging drums 72. Next, the respective color toner is transferred onto the intermediate transfer drum 73 without overlapping the respective color toner, and a print sheet is conveyed from a paper cassette 58, timed with the front end of the image on the intermediate transfer drum 73. The image on the intermediate transfer drum 73 is transferred by a secondary transfer roller 75 onto the print sheet, and the print sheet is discharged onto a discharge tray 62.
However, in this technique, pixel position is not controlled on toner within a 63.5 xcexcm (400 dpi) pixel area. Further, in the printing process, color image formation is made by respective single layer toner, i.e., in a parallel color image structure, positional shift between pixels (color registration) seriously influences degradation of image quality especially in a color space representation area (color gamut).
Further, as another printing technique, toner-jet printing represented by Array Printers AB is known. This technique forms a color image by directly discharging nonmagnetic single-component color toner (black is magnetic single-component toner) on a developing sleeve onto a print sheet. Two-dimensionally-arranged mesh control electrodes are provided between the developing sleeve and the print sheet, and a uniform rear-surface electrode is provided facing the rear surface of the print sheet. The toner is discharged in accordance with print information onto the print sheet by a voltage of 1500 V and a control electrode voltage (275 V in printing and xe2x88x9250 V not in printing) applied between the developing sleeve and the rear-surface electrode. This printing technique has a problem that the hole diameter of the mesh control electrode is about 100 to 150 xcexcm. As toner is discharged in lump, rebound occurs on the print sheet, which results in high background image (the base of the print sheet is stained). This is a weak point of the printing process itself to discharge toner in lump, as well as the rebound phenomenon, and this is the greatest cause of degradation of graininess. The printing technique also has a problem of toner clogging or attachment to the control electrodes. Accordingly, a cleaning unit for the control electrodes is required, which increases the size and price of the apparatus. Japanese Published Unexamined Patent Application No. Hei 4-83658 discloses a technique to apply an alternating voltage to the rear surface of an aperture electrode so as to prevent contamination of the rear surface. In any case, in printing technique to insert a control electrode between a developing sleeve with extremely small gaps about 200 xcexcm and rear-surface electrodes and pass toner through the holes of the control electrodes, the problem of contamination of electrode must be solved.
In printing techniques for color image formation, if particles for image formation are transferred to a position shifted from a desired pixel position on a print medium, such pixel position shift causes degradation of image quality including deterioration of graininess due to scatter or drop of particles and the like, reduction of color-space representation region (color gamut), occurrence of color registration and bonding.
The present invention has been made in consideration of the above situation, and has an object to provide an image printing apparatus which prevents the above degradation of image quality by pixel position control with high precision.
According to one aspect of the present invention, the foregoing object is attained by providing an image forming apparatus comprising: an image holder that forms an image of color material on an image holder surface, the image holder surface dividing into a number of pixel areas each having an adhesive-force change portion smaller than the pixel area where adhesive force to color material changes by a predetermined stimulus; an image-signal input unit that forms an image by distribution of adhesive force to the color material on the image holder surface by inputting a stimulus corresponding to an image signal into the image holder; a color material supply unit that supplies the color material to the image holder surface; and a transfer unit that transfers the image of the color material formed on the image holder surface, directly or via a predetermined intermediate transfer medium, onto an image print sheet on which the image is finally printed.
Further, according to another aspect of the present invention, the foregoing object is attained by providing an image forming apparatus comprising: an image holder that forms an image of color material on an image holder surface, the image holder surface dividing into a number of pixel areas each having an adhesive-substance reception portion, smaller than the pixel area, having adhesive force to color material, to receive substance which disappears by a predetermined stimulus; an adhesive-substance supply unit that supplies substance having adhesive force to color material to the image holder surface such that the adhesive-substance reception portion holds the substance; an image-signal input unit that forms an image by distribution of adhesive force to the color material on the image holder surface by inputting a stimulus corresponding to an image signal into the image holder; a color material supply unit that supplies the color material to the image holder surface; and a transfer unit that transfers the image of the color material formed on the image holder surface, directly or via a predetermined intermediate transfer medium, onto an image print sheet on which the image is finally printed.
Further, according to another aspect of the present invention, the foregoing object is attained by providing an image forming apparatus comprising: an image holder that forms an image of color material on an image holder surface, the image holder surface dividing into a number of pixel areas, each having an adhesive-substance holding portion holding color material supplied to the image holder surface, positioned in each of the pixel areas, and causing change in adhesive force to held color material in accordance with a predetermined stimulus; a color material supply unit that supplies color material to the image holder surface such that the color material holding portion holds the color material; an image-signal input unit that forms an image by distribution of adhesive force to the color material on the image holder surface by inputting a stimulus corresponding to an image signal into the image holder; a color material removing unit that forms an image by distribution of the color material on the image holder surface by removing excessive color material of the color material held on the image holder surface; and a transfer unit that transfers the image of the color material formed on the image holder surface, directly or via a predetermined intermediate transfer medium, onto an image print sheet on which the image is finally printed.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which reference characters designate the same name or similar parts throughout the figures thereof.